1. Field of the Invention
The present invention relates to a display device having an SRAM in a pixel. More specifically, the invention relates to a circuit technology for driving the SRAM.
2. Description of the Related Art
A liquid crystal display device has been developed, which includes a memory element capable of holding video data statically. U.S. Pat. No. 5,712,652 discloses one of basic principles with regard to such a liquid crystal display device. The liquid crystal display device disclosed in U.S. Pat. No. 5,712,652 includes a memory cell in a pixel, and carries out static image display by using binary video data held in the memory cell, accordingly reducing power consumption in the static image display.
In recent years, halftone display or moving image display (referred to as normal display, hereinafter) has been carried out even on the small screen of a cellular telephones. For such form of use, there has been a request made to carry out the static image display at lower power consumption at the time of waiting, and to carry out the normal display by full color at the time of calling. In the case of the liquid crystal display device disclosed in U.S. Pat. No. 5,712,652, however, some improvements must be made to suit such a form of use, as only the static image is targeted to be displayed based on the binary video data.
Now, description will be made for an exemplary constitution, which enables switching between the normal display and the static image display based on the liquid crystal display device of the conventional example disclosed in U.S. Pat. No. 5,712,652.
Hereinafter, video data for black and white display, which is used for the static image display, will be referred to as binary data. Thus, simply written “video data” means full color video data used for normal display.
Each of FIG. 1 and FIG. 2 shows circuitry of one pixel among a plurality of pixels disposed in a matrix form. A pixel 1 includes a pixel portion 4 as a liquid crystal pixel, and an SRAM portion 5 capable of holding binary data.
In the circuitry shown in FIG. 1, video data outputted from a not-shown source driver is supplied through a signal line 3 to the pixel portion 4. Similarly, binary data outputted from a not-shown SRAM driver is supplied through an SRAM writing line 2 to the SRAM portion 5. In this example, the SRAM writing line 2 and the signal line 3 are individually necessary, and the two drivers, i.e., the SRAM driver and the source driver, are also necessary. In FIG. 1 (also in FIG. 2), a reference numeral 6 denotes a scanning line for transmitting a scanning signal outputted from a not-shown gate driver; and a reference numeral 7 denotes an SRAM control line for transmitting a control signal outputted from a not-shown system control circuit.
In another circuitry shown in FIG. 2, two drivers share the signal line 3, and outputs of the drivers are selected by switches 8 and 9. Specifically, in the normal display, the switch 8 is turned on while the switch 9 is turned off, and the output of the not-shown source driver is supplied through the signal line 3 to the pixel portion 4. In the static image display, the switch 8 is turned off while the switch 9 is turned on, and the output of the not-shown SRAM driver is supplied through the signal line 3 and the pixel portion 4 to the SRAM portion 5. In this example, the SRAM writing line 2 shown in FIG. 1 is made unnecessary, but the two drivers, i.e., the SRAM driver and the source driver, are still necessary.
As described above, in the case of the constitution, which enables switching between the normal display and the static image display based on the liquid crystal display device of the conventional example, the SRAM driver is necessary in addition to the source driver. Consequently, the difficulty of reducing manufacturing costs has been inherent.